When springs are manufactured of a high strength stainless steel, punching and forming are involved. Therefore, it is desired that the material is of low hardness before the aging treatment and is of high hardness after the aging treatment.
However, work-hardenable stainless steels represented by AISI 301 steel and precipitation-hardenable stainless steels represented by 17-7 PH steel, which have been conventionally used for manufacturing springs, must be heavily cold-worked in order to enhance hardness after aging. Accordingly, hardness in the cold-worked state before aging is inevitably high. That is, they are defective in that hardness before aging and hardness after aging cannot be separately controlled. These steels are also defective in that difficulties accompany the production thereof and yet adequate hardness after aging cannot be achieved.
Under the circumstances, we previously developed a stainless steel having the composition indicated below which exhibits martensitic structure in the solution-treated state or the lightly worked state and is improved with respect to the above-mentioned defect. This invention is disclosed in Japanese Patent Application No. 34138/80 (Laid-Open Patent Publication No. 130459/81) under the title "Precipitation-Hardenable Stainless Steel for Springs".
The steel contains, in weight percent, more than 0.03% and not more than 0.08% C, not more than 0.03% N, 0.3-2.5% Si, not more than 4.0% Mn, 5.0-9.0% Ni, 12.0-17.0% Cr, 0.1-2.5% Cu, 0.2-1.0% Ti and not more than 1.0% Al, the balance being Fe and inevitable incidental impurities, wherein the amounts of C, Ti, Mn, Ni, Cr, Cu and Al are adjusted so that the value of A' defined as ##EQU1## is less than 42.0, and the amounts of Mn, Ni, Cu, Cr, Ti, Al and Si are adjusted so that the value of Cr-equivalent/ni-equivalent defined as ##EQU2## is not more than 2.7, and that the value of .DELTA.Hv defined as EQU .DELTA.Hv=205.times.[Ti%-3.times.(C%+N%)]+205.times.(Al%-2.times.(N%)]+57.5 .times.(Si%)+20.5.times.(Cu%)+20
is in the range of 120-210, and the steel exhibits substantially martensitic structure in the solution-treated state or in the not-more-than 50% cold-worked state.
This previously developed steel is excellent in punching and forming workability and it exhibits satisfactory properties as a spring material when .DELTA.Hv (the difference in hardnesses before and after aging) is adjusted to around 200. This steel can be easily produced since heavy cold working is not required.
In comparison with the maraging steels represented by 18 Ni maraging steel, however, this steel is slightly inferior in toughness when used for springs or for constructions in the domain of the high strength steel (around 190 kg/mm.sup.2 in notch tensile strength).
We studied for improving toughness of this previously developed steel and we have found that toughness of the steel can be retained at high strength by addition of Mo. That is, we have found that toughness of the material can be well retained by addition of Mo even if .DELTA.Hv (degree of aging), which was restricted to not more than 210 in consideration of toughness in the previously developed steel, is raised to more than 210. Also we have found that enhancement of strength can be attained by addition of Mo without depending upon the precipitation hardening effect of Cu, except when Cu is necessary for improvement of corrosion resistance against the sulfurous atmosphere.